Mobile communications systems such as mobile telephones and portable telephones adopt TDAM (Time Division Multiple Access) system, so that they may efficiently use communications lines as communications systems between a base station and radio devices which operate as mobile terminals.
Some specific examples of the mobile communications systems adopting the TDMA system are PDC (Personal Digital Cellular System; Japan), PHS (Personal Handyphone System; Japan), and GSM (Global System for Mobile Communications; Europe).
In these systems, communication signals or burst signals are exchanged between the base station and each radio device (i.e., a mobile station). As FIG. 5 shows, each burst signal has a signal region T.sub.1 and a non-signal region T.sub.2 in every signal cycle T.sub.0.
The burst signal has been modulated with digital data which is set in a transmission frame to be transmitted.
To provide more communications lines between the base station and the radio devices (mobile stations), narrow carrier frequency bands are allocated to the adjacent communications lines.
As described above, the carrier is modulated with the digital data to be transmitted. The modulated signal therefore contains not only a component of frequency f.sub.c but also many other components of frequencies near frequency f.sub.c.
As a result, the frequency range for the modulated signal broadens. If the frequency range for the components of the modulated signal broadens, the component of communication signal a to be transmitted on the carrier will acts as leakage power, adversely influencing adjacent communication signal lines on adjacent carriers.
The influence of the leakage power on the adjacent carriers, i.e., the adjacent channels, is regulated by the standards of each system.
ETS300 607 of ETSI for the GSM system is one of such standards.
According to the standards for the GSM system, the influence is regulated in terms of a fall (dB) of each power level at a frequency, which differs, from the carrier frequency f.sub.c by an offset frequency f.sub.OFF.
Apparatuses designed for use in NADC (North America Digital Cellular) systems to measure leakage power in the communication signal composed of a burst signal are known. One example is an adjacent and alternate channels power measurement apparatus shown in FIG. 6 (Jpn. Pat. Appln. KOKAI Publication No. 6-326672; U.S. Pat. No. 5,475,709).
An input signal a.sub.1 composed of a burst signal of the type shown in FIG. 5 is input from an input terminal 1 to the mixer 3 of a signal processing section 2.
The mixer 3 converts the input signal a, to an intermediate-frequency signal (IF signal), by using a local oscillation signal supplied from a local oscillator 4. A band-pass filter 5 removes unnecessary frequency components from the intermediate-frequency signal. Further, a level detector 6 detects the envelope of the signal, and an A/D converter 7 converts the signal to a digital input signal.
A frequency switching section 8 switches the frequency of the local oscillation signal supplied from the local oscillator 4, to a frequency which corresponds to the self channel, i.e., the carrier frequency f.sub.c of the input signal a.sub.1. The section 8 then sequentially switches this frequency to frequencies, which correspond to the channels adjacent to the self-channel, i.e., the carrier frequencies of the adjacent channels.
Hence, the signal processing section 2 outputs digital input signals of the frequency components contained in the self channel and adjacent channels, one after another, in synchronism with the switching of the local oscillation frequency performed in the frequency switching section 8.
A data writing section 9 receives the input signals of the self-channel and the adjacent channels which have been sequentially output from the signal processing section 2. The section 9 then writes these signals into the regions of a waveform memory 10 which are assigned to the channels, respectively.
A data reading section 11 detects the rise timing of the envelope waveform stored in that region of the waveform memory 10 which is assigned to the main channel. That is, the section 11 detects the address of the rise timing in the signal region T.sub.1 of the burst signal.
Using the timing detected, the reading section 11 reads the digital input signals of the main channel and adjacent channels stored in the regions of the waveform memory 10 and supplies these signals to a power calculating section 12.
The power calculating section 12 calculates power from the signals of each channel, which have been input from the waveform memory 10 and whose band has been limited. The section 12 outputs the powers of the adjacent channels as leakage powers.
With the adjacent and alternate channel power measurement apparatus shown in FIG. 6, however, there are some object that need to be attained.
The objects are concerned with the method of measuring the leakage powers of adjacent channels, which should be performed in the GSM system described above.
The method of measuring the leakage powers of adjacent channels, which should be performed in the GSM system, comprises two steps. First, signals are received at the self-channel and the adjacent channels by using a filter having a bandwidth of 30 kHz. Second, the average power of the specified region of each signal is obtained.
FIG. 7A shows the waveform of the envelope power as viewed along the time axis, which is observed when a communication signal is received from a GSM system in the band width of 30 kHz.
FIG. 7B represents the waveform observed when a GSM signal is received in a bandwidth of 1 MHz.
As seen from FIG. 7B, the rising position and falling position of the burst signal received in the broad band can be detected with high accuracy.
As shown in FIG. 7A, however, it is difficult to detect the rising position and falling position of the burst signal received in the narrow band. This is because the filter response influences the waveform.
In the case shown in FIG. 7A, the signal may have no peaks at the rising position or the falling position, unlike in the case illustrated in FIG. 7B.
In view of this, the average value of the signals should be presented, instead of the peak values thereof, in the method of measuring the leakage powers of adjacent channels. If so, the signals will differ but less from one another, making it easier to evaluate them.
In the GSM system, evaluation is made on the basis of both the peak value and the average value. It is instructed that the average value be calculated for a specific region.
The specific region designated in the GSM system is the region (T3) which extends from the latter half of the burst signal, which excludes the training sequence, i.e., synchronizing word part, of a fixed pattern located close to the center of the burst signal, to the part which is not influenced by a transient response.
More specifically, 40 bits or more, or bits in the range of the 87th bit to 132nd bit, are designated to serve as the specific region.
In the adjacent and alternate channels power measurement apparatus shown in FIG. 6, however, the signal output from the band-pass filter 5 has such a waveform as is illustrated in FIG. 7A.
With the position-determining method which is employed in the conventional apparatus shown in FIG. 6 and which uses the rising position and falling position of a burst signal, it is difficult to find the position of the specific region to meet the demand made by a GSM system. Therefore, an average power for the specific region cannot be calculated.
That is, the conventional apparatus shown in FIG. 6 is designed to accomplish adjacent and alternate channels power measurement in the NADC (North America Digital Cellular) systems.
In the case of NADC, the band measured is almost the same as the band occupied, and it is used in communication. The band itself is not so broad, having a width of 25 kHz. Demodulation can therefore be achieved even after the frequency has been changed and the band has been limited.
Hence, in the NADC-adapted measuring apparatus it is possible to use a band-limiting filter before the signals are stored into the memory.
By contrast, an apparatus designed to achieve adjacent-channel measurement in the GSM (Global System for Mobile Communications; Europe) cannot perform demodulation after the band has been limited. This is inevitably because the GSM measuring-band is narrow, having a width of 30 kHz which is narrower than the band for use in communication (having a width of about 300 kHz).